The invention relates to a method of scanning indicia using selective sampling, and particularly although not exclusively to a method of reading bar codes using a laser scanner.
Various optical readers and optical scanning systems have been developed heretofore for reading bar code symbols appearing on a label or on the surface of an article. The bar code symbol itself is a coded pattern of indicia comprising a series of adjacent bars and spaces of various widths, the bars and spaces having different light reflecting characteristics.
A number of different bar code standards or symbologies exist. These symbologies include, for example, UPC/EAN, Code 128, Codabar, and Interleaved 2 of 5. The readers and scanning systems electro-optically decode each symbol to produce multiple alphanumerical characters that are intended to be descriptive of the article or some characteristic thereof. Such characters are typically represented in digital form as an input to a data processing system for applications in point-of-scale processing, inventory control, and the like. Scanning systems of this general type have been disclosed, for example, in U.S. Pat. Nos. 4,251,798; 4,360,798; 4,369,361; 4,387,297; 4,409,470 and 4,460,120, all of which have been assigned to Symbol Technologies, Inc., the assignee of this application. As disclosed in some of the above patents, one commonly used example of such a scanning system functions by scanning the laser beam in a line across a symbol. The symbol, composed of alternating, rectangular, reflective and non-reflective segments of various widths, reflects a portion of this laser light. A photo detector then detects this reflected light and creates an electrical signal indicative of the intensity of the received light. The electronic circuitry or software of the scanning system decodes the electrical signal creating a digital representation of the data represented by the symbol scanned.
Typically, a scanner includes a light source such as a gas laser or semiconductor laser that generates a light beam. The use of semiconductor lasers as the light source in scanner systems is especially desirable because of their small size, low cost and low power requirements. The light beam is optically modified, typically by a lens, to form a beam spot of a certain size at a prescribed distance. It is preferred that the beam spot size be no larger than approximately the minimum width between regions of different light reflectivities, i.e., the bars and spaces of the symbol.
A scanner also includes a scanning component and a photo detector. The scanning component may either sweep the beam spot across the symbol and trace a scan line across the past the symbol, or scan the field of view of the scanner, or do both. The photodetector has a field of view which extends across and slightly past the symbol and functions to detect light reflected from the symbol. The analog electrical signal from the photodetector is first typically converted into a pulse width modulated digital signal, with the widths corresponding to the physical widths of the bars and spaces. This signal is then decoded according to the specific symbology into a binary representation of the date encoded in the symbol to the alphanumeric characters so represented.
In the prior art described above, a digitizer circuit may be used to translate the analog signal into a digital representation called a Digital Bar Pattern (or DBP). This simple digital representation of the data works extremely well in many situations, although it may sometimes be susceptible to unrecoverable errors if the bar code symbol to be read has substantial noise associated with it. With this prior art representation, a single extra edge detected or shifted due to noise may prevent proper decoding.
One straightforward way to acquire a more accurate representation of the bar code, for example for more aggressive or adaptive decoding, would be to sample the analog signal above the Nyquist rate, store the analog signal in memory, and then apply digital signal processing (DSP) techniques. This solution is, however, very expensive due to the large amount of samples required and the high speed processing that is necessary.
There is accordingly a need to provide a relatively cheap and reliable method of decoding an indicia (for example a bar code Symbol) after the optical detection system has transduced it into a distorted analog waveform. Such a need is particularly acute where it is desired to decode the symbol aggressively, that is by attempting to decode after a single scan.
It is an object of the present invention to aim to meet this need.
It is a further object of the invention to provide an efficient and economical means of acquiring an improved representation of the bar code signal.